If he'd been in Aria longer, he would have probably had an event that jarred him enough that he needed mom/adult support. Honestly, being lost in another new world and stuck away from Aria and ending up in the Meadous could probably be it. If Toriel is the nicest person that he's encountered thus far, and she's mom'd at him effectively, he might end up having his meltdown at her. It wouldn't be surprising to me at all Everywhere I have ever played him, he has kept himself together for quite a while until he just hasn't anymore...and the meltdowns can sometimes be pretty ridiculous.

He would appreciate Toriel's fussing. Even if he sometimes pretended to not like it. But it would make him even more homesick for his actual mother, which I'm sure she'd notice.